Submersible pumping systems are often used in hydrocarbon producing wells for pumping fluids from within the well bore to the surface. These fluids are generally liquids made up of produced liquid hydrocarbon and often water. One type of system used in this application employs an electrical submersible pump (“ESP”). ESP's are typically disposed at the end of a length of production tubing and have an electrically powered motor. Often, electrical power may be supplied to the pump motor via an electrical power cable from the surface that is strapped alongside the tubing.
ESP motors have stators with axially oriented slots and insulated magnet wires wound through the slots in a selected pattern. A sheet of an insulation material is usually wrapped around each bundle of magnet wires within each of the slots. The magnet wires extend below a lower end of the stator in loops spaced around a longitudinal axis of the motor. The magnet wires may be bonded in the slots with an epoxy resin to resist mechanical vibration during operation. In one technique, magnet wire leads are spliced to upper ends of three of the magnet wires. The magnet wire leads extend from the upper end of the stator to internal contacts in a motor electrical plug-in receptacle. A dielectric lubricant fills the motor for lubricating bearings within the motor.
Typically, the pumping unit is disposed within the well bore just above where perforations are made into a hydrocarbon producing zone. In this position the produced fluids flow past the outer surface of the pumping motor and absorb heat generated by the motor. In spite of the heat transfer between the fluid and the motor, the motor may still overheat. Overheating may be a problem when the fluid has a high viscosity, a low specific heat, or a low thermal conductivity. This is typical of highly viscous crude oils. Also, the motor may be forced to operate at an elevated temperature past its normal operating temperature in steam injection wells. Elevated well temperatures can reduce motor life. Undesirable chemicals may be formed when the epoxy resin degrades under high temperature. These chemicals can damage the insulation layers of the magnet wires.